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Art. XXXVII. — On the Geology of the North Head of Manukau Harbour. 

 By Captain F. W. Hutton, F.G.S. 



(With Illustrations.) 



[Bead before the Auckland Institute, August 16, 1869.] 



The cliffs, north of Manukau Harbour, are composed of a coarse volcanic 

 agglomerate of various kinds of dolerite, trachyte, and rhyolite, the trachytic 

 rocks, however, being mixch the most numerous. This agglomerate is gener- 

 ally horizontal, but at Paratutai — the rock that forms the north head of the 

 harbour — it is seen to dip strongly to the north, and on its upturned edge rests 

 a thin bed of vescicular doleritic lava, covered, conformably, with beds of 

 agglomerate, to which succeeds another lava stream, also covered by 

 agglomerate. The conformability of these doleritic rocks with the rest of the 

 formation, together with their vescicular character, makes me class them as 

 lava streams, instead of dykes. Proceeding northwards along the coast, 

 several dykes of doleritic-trachyte, very similar to the lava streams already 

 mentioned, but more compact, are seen, cutting through the agglomerate. The 

 second of these, or the one first seen after passing the valley that divides the 

 hill, on which the old pilot station was placed, from the rest of the cliffs, is 

 about six feet thick at the base, and dips 80° S.E., but rapidly thins out 

 upwards, and comes to an end less than two-thirds of the way up the cliff, 

 showing that it has been injected from below, but had never reached the 

 surface. Besides these true dykes there are also other reefs of rock which at first 

 sight look like dykes, but, on a closer examination, are seen to be fissures 

 filled up with fine-grained tufa, of the same composition as the matrix of the 

 surrounding agglomerate. These fissures were perhaps caused by earthquakes 

 at the time when the volcanic forces were in activity, and may help us to 

 understand the original formation of some of the lodes at the Thames. 



Proceeding further northwards, at a distance of about a mile and a half 

 from Paratutai, we find that the lower part of the formation has been thrown 

 up by a fault, and is seen to rest upon beds of fine-grained tufa, tufaceous 

 sandstone, and sandstone, which, no doubt, belong to the upper part of the 

 Waitemata series ; for similar rocks occur at Puponga, as described by Dr. 

 Hochstetter. 



In the hill, under the old pilot station, a large angular mass of fine- 

 grained tufaceous sandstone, intei^stratified with beds of shale, is seen, enclosed 

 in the agglomerate. (Pl.IXa. Fig. I.) This mass is about twenty-five by fifty feet, 

 and probably weighs not less than 2000 tons ; it belongs to the underlying 

 Waitemata beds, and must have been thrown up by a volcano. That this 

 volcano must have existed in the close neighbourhood, is proved by the large 

 size of the block, as well as by the lava streams at Paratutai, although no 

 trace of it can be now recognised ; and the fact that the block, although 

 composed of fragile materials, was not shattered in pieces, proves that it was 

 ejected under water. On the eastern, or inner, side of Paratutai the cliff is 

 being undermined and worn away at low-water mark (Fig. II.), while at high- 

 water mark, or a little above it, another, and older, undermining of the sea 

 can be observed, forming a terrace, the difference of height between the two 

 being about ten feet, showing that the land has here risen that distance since 

 the higher one was formed. This closely corresponds to the height of the 

 raised beach at the Thames, on which Shortland and Grahamstown are built. 

 On the outer, or west, side of Paratutai, a similar terracing exists, as can be 

 seen in the Rev. J. Kinder's photograph ; but I was not able to measure it, 

 and so am unable to say whether the two are at equal heights. 



